3 ms·

there are a couple pretty core assumptions that make that hard: read() interfaces expect the kernel to put the data where you asked. its hard but not impossibl

there are a couple pretty core assumptions that make that hard: read() interfaces expect the kernel to put the data where you asked. its hard but not impossible to coordinate with the NIC to demultiplex the packet and get it into the right place without a copy. almost all of the time you just want to see the data, you dont care that it lands in this particular spot. getting control flow in and out of the kernel is expensive. target address need to be checked. registers often need to be saved. things like the kernel firewall take a certain amount of work to determine the packet status there are lots of mitigations, but at some point you run into a conflict between general purpose multi-process functions and performance. if you ran your application on a dedicated arm next to the nic I bet you'd see a difference. maybe that shouldn't be so hard.

CoolGuySteve 8y ago
That still doesn't add up though. System call overhead is something like 500ns to 1usec depending on the call. Copying a large 1500 byte UDP MTU from ram is pessimistically along the lines of 2.5 usec (1500 bytes / 64 byte cacheline * 100ns L3 access). So a simple UDP recv() + copy from network card buffer to userspace should be about 3-4usec. In the real world, it's more like 6 or 7usec the last time I timed it. Admittedly, the unix API could use a refresh. A lot of unix calls like getaddrinfo are perfectly happy to allocate and give you a buffer. A "fast_recv" could easily do the same and just hand you a page from a kernel page pool directly that you then have to free. By avoiding the buffer copying, you could get something in the same ballpark as kernel bypass but without all the bespoke code and frameworks in every application. And in the end, that's kind of the point of having an "Operating System" in the first place.
- ignoramous 8y ago
  > So a simple UDP recv() + copy from network card buffer to userspace should be about 3-4usec. In the real world, it's more like 6 or 7usec the last time I timed it. Not sure where the Linux kernel lags (may be at scale it kind of tails out?)-- if you read Google's paper on their network load balancer, Maglev, where section 5.2.1 specifically calls out 30% performance degradation without bypass on smaller packet sizes. > By avoiding the buffer copying, you could get something in the same ballpark as kernel bypass but without all the bespoke code and frameworks in every application. May be this article from Cloudflare helps paint a proper picture of why certain kind of applications may need to keep bypassing the kernel forever because their requirements are very specific and not because the kernel can't be made to go faster (ex: a virtual router/switch, or a load-balancer): https://blog.cloudflare.com/why-we-use-the-linux-kernels-tcp-stack/ https://blog.cloudflare.com/why-we-use-the-linux-kernels-tcp...

CoolGuySteve 8y ago

That still doesn't add up though. System call overhead is something like 500ns to 1usec depending on the call. Copying a large 1500 byte UDP MTU from ram is pessimistically along the lines of 2.5 usec (1500 bytes / 64 byte cacheline * 100ns L3 access). So a simple UDP recv() + copy from network card buffer to userspace should be about 3-4usec. In the real world, it's more like 6 or 7usec the last time I timed it. Admittedly, the unix API could use a refresh. A lot of unix calls like getaddrinfo are perfectly happy to allocate and give you a buffer. A "fast_recv" could easily do the same and just hand you a page from a kernel page pool directly that you then have to free. By avoiding the buffer copying, you could get something in the same ballpark as kernel bypass but without all the bespoke code and frameworks in every application. And in the end, that's kind of the point of having an "Operating System" in the first place.

ignoramous 8y ago
> So a simple UDP recv() + copy from network card buffer to userspace should be about 3-4usec. In the real world, it's more like 6 or 7usec the last time I timed it. Not sure where the Linux kernel lags (may be at scale it kind of tails out?)-- if you read Google's paper on their network load balancer, Maglev, where section 5.2.1 specifically calls out 30% performance degradation without bypass on smaller packet sizes. > By avoiding the buffer copying, you could get something in the same ballpark as kernel bypass but without all the bespoke code and frameworks in every application. May be this article from Cloudflare helps paint a proper picture of why certain kind of applications may need to keep bypassing the kernel forever because their requirements are very specific and not because the kernel can't be made to go faster (ex: a virtual router/switch, or a load-balancer): https://blog.cloudflare.com/why-we-use-the-linux-kernels-tcp-stack/ https://blog.cloudflare.com/why-we-use-the-linux-kernels-tcp...